fix(compact): size the compaction e2e to actually cross threshold; sync stale docs (CBR-007)

The compaction e2e never exercised compaction: its window/fixture combo
(contextWindow 8000, thresholdRatio 0.5 → threshold 4000; four small files)
peaked at ~1389 estimated tokens, so compactIfNeeded declined every pre-step
and compact/start never landed. Shrink the window (contextWindow 2400 →
threshold 1200; retainTokens 500 + summarizationMaxTokens 300 = 800 < 1200,
convergence holds) and grow the fixture to six files so a couple of bash steps
reliably cross the threshold. Verified compaction fires and the suite passes
across repeated real-API runs.

Sync docs left stale by the landed compaction work: list compaction.e2e.ts and
keyless-smoke.e2e.ts in the coding-agent README (and fix the wrong "Both
self-skip" count), add compaction to the examples with-key inventory, and
replace the hypothetical compaction/marker / "future plugin" naming in the
session README, session types JSDoc, and the core-data-structures catalog with
the real compact/start, compact/summary, compact/end events.

packages/core/session/src/types.ts

@@ -156,7 +156,8 @@ export type TurnEndReason = TurnEndReasonMap[keyof TurnEndReasonMap]
  * same events; trace/telemetry = subscribe to the log.
  *
  * Merge-extensible: plugins declare extra event types via declaration merging
- * (e.g. a compaction plugin adds `'compaction/marker'`).
+ * (e.g. the compaction plugin adds `'compact/start'`, `'compact/summary'`,
+ * `'compact/end'`).
  *
  * Durability contract (what a persistence backend relies on): the durable log
  * persists every event verbatim, INCLUDING `assistant/chunk` — `seq` must stay
@@ -279,7 +280,7 @@ export type SessionEvent<T extends SessionEventType = SessionEventType> = {
     /**
      * Seq numbers of events that are provenance sources of this event
      * (e.g. the `assistant/chunk` seqs that built an `assistant/message`,
-     * or the surface nodes shadowed by a compaction marker).
+     * or the surface nodes shadowed by a compaction replace node).
      */
     sourceEventSeqs?: number[]
     /** How this event entered the surface; absent for non-surface events. */